arch/riscv: Remove enter_supervisor

This function is unused since coreboot starts payloads in machine mode,
and it uses the obsolete eret instruction.

Change-Id: I98d7d0de5a3959821c21a0ba4319efb610fdefde
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15729
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
2 files changed